UK Monster Owners Club Forum » .: Technical :. » Mods & How To's » Crankcase breather

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Old 11-03-2020, 09:39 AM   #1
Goofle
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Crankcase breather

I’m swapping my standard Monster carbs for Keihin 41mm FCRs (purely because I’ve always wanted a bike with flat slides!). Were I to swap the standard air box (albeit drilled for increased flow) for pod filters (with a suitable battery box and mount for the igniters) would a simple cone filter for the crankcase breather suffice? That’s my experience previously when binning an air box...
And if anyone has experience or opinions here please shout up!
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Old 11-03-2020, 11:55 AM   #2
utopia
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My 750 has had the crankcase breather disconnected from the airbox for over 25k miles, but I retained the separator box and just replaced the pipe connecting that to the airbox with a small filter.
Thus the breather system as a whole is not substantially different from the original set-up and its function is not altered or impaired.
I've had no issues with it whatsoever.

I also have some FCRs awaiting fitment but I intend to retain the std air filter and airbox.
I have little experience of pod filters but I'm wary of their performance being hampered if they get a soaking in the rain.
I also prefer to retain the supply of relatively still and turbulence-free air to the carbs provided by the airbox.
But as I said, I have little or no practical experience to back up these views.
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Old 11-03-2020, 12:44 PM   #3
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I didn't want pod filters on my 900 so have gone down the MWR route - expensive but combines an airbox lid and filter in one item so retains the stock airbox.
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Old 11-03-2020, 01:22 PM   #4
350TSS
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Just relaying what I have read/heard elsewhere. CV (Mikuni) carbs need still air to work well and they also need an unobstructed bell mouth as pressure fluctuations impact on the diaphragm and hence the mid range fuelling. Hence the need for the air box and why pod filters with the obstruction of the end cap do not necessarily provide any performance advantage for Mikuni CV carburetors.
I have read also elsewhere that most pod filters are not actually very good at filtering the air, they pass more air when clean because the gaps between the mesh are much bigger so harmful stuff is more likely to get through them. Google "effectiveness of different types of air filters".
Flat slides with no diaphragms do not suffer the same issues around calmness of the air entering them, though there may still be performance downsides from pod filters by virtue of the restriction caused by the end cap of the pod filter.
Foam filters are what I shall be using as the air box on my M900 is history, about the same filtration capability without the restrictive end cap. (Demon Tweeks)
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Old 11-03-2020, 06:07 PM   #5
Goofle
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Thanks to all who’s replied - I’ll give it some thought then. The engine is standard, the fcr’s are set up for the airbox being fitted but with a very well drilled lid. Mmmm, maybe there’s not much point here in swapping after all other than maybe better induction noise..
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Old 11-03-2020, 06:38 PM   #6
Darren69
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I dont think there's any advantage dirching the airbox even with FCR's. You wont gain much if any and filtration probably not as good. Fit an open airbox lid or chop top off of yours. You'd be better off changing the exhaust system to a 45 to 50mm spaghetti system unless you have already done so as thats more restrictive and will give you the best results with the FCR's imho.
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